This invention relates to printers, and more particularly, to control systems for printers.
Printers experience periodic vibrations that detrimentally impact print quality. Consider, for example, a shuttle-type printer that has a carriage system to move and position print elements (e.g., printheads, replaceable cartridges, etc.) over a printing surface. The carriage system includes a carriage that carries the print elements and a drive assembly that moves the carriage along a rod to position the carriage precisely over the printing surface. Some carriage systems also have anti-rotation rollers or sliders that prevent rotation of the carriage around the rod.
Modern shuttle-type printers, such as inkjet printers, operate at high resolution and position errors as small as 5-10 microns. At this level of precision, vibrations can cause noticeable defects in print quality. While it is desirable to reduce or eliminate such defects, a countervailing design requirement is to improve throughput and printing speed. This requirement typically results in higher carriage velocities, which unfortunately makes it more difficult to reduce the vibration-induced errors.
The carriage system is a source of many periodic vibrations. The carriage system is a mechanical assembly having a characteristic mass and drive belt stiffness. Energy sources excite a natural frequency in the mechanical assembly. In addition, high accelerations and slew rates in the carriage system result in vibrations of the carriage, the rod, and/or chassis.
The periodic vibrations can cause periodic displacement of dots formed on the recording media that result in noticeable defects in the printed image. Such defects include vertical or diagonal bands of different hues. Color hue is significantly impacted by the amount of overlap of neighboring drops. The magnitude of error required to cause hue defects is proportional to the size of the dot formed on the page. If very small drops (e.g., 8-10 picoliter drops) are deposited on a coarse grid (e.g., 600 dpi or 42-micron spacing), the resulting grid has thin areas of white space between dot rows. Periodic vibrations cause displacement of the dots into the white space, yielding significant and noticeable hue shifts.
The wavelength of the hue bands due to vibrational errors is equal to the carriage slew rate divided by the vibration frequency. The spacing between nozzle columns on the printhead and the spacing between printheads of different colors result in peak error amplitudes for like colors occurring at the same horizontal spacing across the page. The peak amplitudes for different colors occur at different positions across the page, resulting in vertical or nearly vertical bands of different hues.
Ideally, the magnitude of vibrational errors is reduced to a point that hue shifts due to vibrations are imperceptible to the human eye. However, as dot size and resolution are reduced, the magnitude of the errors must also be reduced. The expense and difficulty of reducing vibration amplitude increases substantially at levels below 10 microns. Mechanical solutions, such as stiffened structures, damping, and minimization or isolation of input energy, are effective at reducing vibration amplitude, but often come at a prohibitively high cost and/or contradict high acceleration or slew rate.
Accordingly, there remains a need for a simple and cost-effective approach to reducing print defects caused by periodic vibrations in the printer.
This invention concerns techniques for masking the effects of periodic vibrations in printers. The techniques do not eliminate or reduce the vibrations themselves, but mask their effects in the print results to reduce the visibility of vibration-induced errors.
In one implementation, a shuttle-type printer has a movable carriage to carry and position at least one pen over a printing surface. The printer also has a carriage positioning system coupled to move the carriage at a slew rate in swaths over the printing surface. The carriage positioning system has a predefined starting position for individual swaths.
According to one technique, the carriage positioning system varies the starting position for the individual swaths. According to a second technique, the carriage positioning system makes multiple passes before advancing the recording media and varies the slew rate for individual passes. Each of these techniques reduces visual artifacts in the print results that are caused by period vibrations in the printer.